Blended powders of molybdenum and self-fluxing NiCrFeBSi alloys are plasma sprayed onto metal surfaces to produce wear resistant coatings. Typical applications include mechanical parts subject to contact sliding conditions such as the piston rings and cylinder liners of internal combustion engines. In general, these blends consist of spray dried or densified molybdenum powder and atomized NiCrFeBSi alloys. An example of this type of thermal spray powder is described in U.S. Pat. No. 3,313,633. Unfortunately, coatings made from these powders exhibit rapid degradation and increased friction coefficients once the wear process is initiated. In particular, the degradation of these coatings is accelerated by coating break out failures, e.g. coating particle pull out and delamination of coating layers. These types of failures lead to increased friction between contacting surfaces and hence increased wear. Oxidation of the molybdenum during spraying is believed to be a principal cause of these types of failures.
U.S. Pat. No. 4,597,939 to Neuhauser et al. describes using a thermal spray powder containing a blend of molybdenum, molybdenum carbide and 80/20 NiCr alloy powders to produce a tougher plasma sprayed coating which is less prone to coating break out. The NiCr alloy component is employed to increase the toughness of the coating and the molybdenum carbide to provide the wear resistance. However, because of the relatively low hardness of the NiCr alloy, these powders produce coatings having low hardness values and consequently less wear resistance than the coatings made with the self fluxing NiCrFeBSi alloys.
U.S. Pat. No. 5,063,021 describes a method for preparing a thermal spray powder in which a blend of molybdenum and self-fluxing alloy powders is pre-alloyed through sintering and plasma densification prior to plasma spraying. However, the thermal spray powders prepared by this method exhibit poor sprayability in piston ring applications, producing coatings which have considerable porosity and poor adhesion.
Thus, it would be a distinct advantage over the prior art to provide a thermal spray powder which would increase the resistance of thermal spray coatings to coating break out, while providing high wear resistance and retaining sprayability.